1. Field of the Invention
This invention relates generally to oscillators and, more specifically, to a piezoelectric based oscillator which consumes less power and which has faster start-up times than prior art oscillators.
2. Description of the Prior Art
The start-up time of an oscillator is defined as the time required for the oscillator to reach a steady state. Presently, for most oscillators, the start-up time can be several seconds depending on the crystal frequency and amplifier design of the oscillator. The start-up time may be even longer when the temperature of the device using the oscillator increases. This creates a problem since the start-up times may be fairly long and unpredictable.
The reason for the delay is that when the oscillator circuit is powered up, the output of the amplifying inverter begins to bias the input through a bias resistor. The bias resistor and the load capacitors are large and the amplifying inverter may be weak. This causes considerable delay for the oscillator circuit to reach appropriate bias levels.
Once the amplifying inverter of the oscillator circuit is biased to its region of maximum gain, the amplifying inverter is unstable. The oscillator circuit relies on thermal noise to provide excitation energy at the piezoelectric resonator's resonant frequency. The problem with thermal noise is that thermal noise has low energy, varies with operating conditions, and is white noise which means that the noise is equally distributed over a given frequency band. Thus, there is just as much energy at the piezoelectric resonator's overtone and spurious frequencies as there is at the piezoelectric resonator's fundamental frequency.
After start-up, the oscillator circuit losses cause the oscillator circuit to stabilize (i.e., loop gain is approximately one). The oscillator circuit then enters a steady state operating mode. However, high gain was required to ensure a fast and reliable start-up. This creates a problem since in steady state operation, the high gain wastes power. Furthermore, the high gain may result in piezoelectric resonator overdrive.
Therefore, a need existed to provide an improved oscillator. The improved oscillator must have faster start-up times than present oscillators. The improved oscillator must further have faster and more predictable start-up times even though operating conditions may fluctuate and change. The improved oscillator must further provide excitation energy at the piezoelectric resonator's resonant frequency once the amplifying inverter is biased to a region of maximum gain, without relying on thermal noise. The improved oscillator must provide excitation energy at the piezoelectric resonator's resonant frequency by injecting a high energy noise pulse whose spectral energy decreases with increasing frequency. The improved oscillator must reduce power consumption by lowering the high gain that was required for a fast reliable start-up. The improved oscillator must monitor the output of the oscillator circuit to determine when steady-state operation has been achieved and then lower the gain of the oscillator circuit.